1. Field of the Invention
The present invention relates in general to a fuel tank vapor control system for an automotive vehicle and more particularly to a means for warning of malfunction of a canister in a fuel tank vapor control system.
2. Description of the Prior Art
As is well known, a fuel tank vapor control system for emission control of an automotive vehicle, includes a canister of activated charcoal, which is connected to a fuel tank and adsorbs or traps evaporative hydrocarbons or hydrocarbon vapors from the fuel tank for thereby preventing them from escaping to the atmosphere (refer to Japanese Patent Provisional Publication No. 58-148259).
However, the canister has a possibility of malfunctioning and becoming incapable of trapping the hydrocarbon vapors from the fuel tank or feeding them back into the engine. For this reason, a device for monitoring the operating condition of the canister has been required.
An example of such a device includes a temperature sensor installed on the canister for detecting a temperature rise and temperature fall of the canister resulting from adsorption and regeneration of the hydrocarbon vapors and is adapted to judge that the canister is malfunctioning when such a temperature fall cannot be detected.
However, in the event of a high-load engine operating condition in which an associated vehicle is, for example, climbing an uphill under an atmospheric temperature condition of 30.degree. C., a quantity of hydrocarbon vapors supplied from the fuel tank to the canister increases sharply to cause the canister to be put into an overflow condition in which a flow rate of the hydrocarbon vapors supplied from the fuel tank to the canister (i.e., charge flow rate) is larger than a flow rate of the hydrocarbon vapors regenerating or coming away from the canister (i.e., purge flow rate). Under this overflow condition, the weight of the contents of the canister (i.e. weight of hydrocarbonlaiden charcoal) is maintained substantially unchanged, i.e., in an equilibrium state while at the same time the inside temperature of the canister (i.e., temperature of charcoal) is maintained substantially unchanged, i.e., in an equilibrium state. Due to this, with the prior art device, such an overflow condition is erroneously judged as malfunction of the canister.
On the contrary, when the charge flow rate is so small, a variation of the inside temperature of the canister does not almost occur even when the canister is functioning properly. With the prior art device, such a small charge rate condition is also judged as malfunction of the canister.